The present invention relates to a shift control system for an automatic transmission for motor vehicles, and more particularly, to hydraulic-pressure control of an engagement-side friction engaging element.
The automatic transmission comprises generally a shift mechanism including a planetary-gear set wherein coupling or fixing of a sun gear, a carrier, and the like is carried out by engaging or releasing hydraulic friction engaging elements such as a wet multiple-disc clutch, achieving a desired gear. A torque converter or fluid coupling comprising a pump on the input side and a turbine on the output side is interposed between an internal combustion engine and the shift mechanism. The torque converter increases and transmits torque of the engine at vehicle start, and absorbs shock due to transferred torque at shifting, quick acceleration/deceleration, and the like.
A large number of recently commercialized shift mechanisms are of the electronically controlled type wherein solenoid valves for controlling the hydraulic pressure are duty-controlled by an electronic control unit (ECU) to release and engage the friction engaging elements. Typically, the automatic transmission having such shift mechanism carries out shift control in accordance with a shift map having throttle opening and vehicle velocity as parameters. Specifically, a shift command is provided at an instant when the cruising condition corresponds to downshift timing or upshift timing on the shift map, in accordance with which the hydraulic pressure supplied to the engagement-side friction engaging element or the hydraulic pressure discharged from the release-side friction engaging element is controlled to carry out gear change.
In this shift control, an initial value of hydraulic pressure supplied to the engagement-side friction element, i.e. the starting supply pressure, is set in accordance with turbine torque obtained from engine torque. During shifting, the duty ratio of the solenoid valves is feedback-controlled at an optimum value to obtain appropriate hydraulic pressure for prompt close of that shifting.
In this feedback control, a target rate of change of turbine rotational speed is determined in accordance with a previously set shift time and a predicted difference in turbine rotational speed. The hydraulic pressure is increased or decreased so that an actual rate of change of turbine rotational speed determined based on actual measurement approaches the target rate of change. With this, favorable shifting is achieved without having occurrence of simultaneous engagement or release of the engagement-side and release-side friction engaging elements.
In order to stabilize feedback control, JP-A 8-145157 describes a learning correction of the starting supply pressure in accordance with a deviation between the target rate of change of turbine rotational speed at an initial stage of shifting and the actual rate of change of turbine rotational speed which varies with the cruising condition. Moreover, the reference proposes a technique on upshift (power-on upshift) control carried out when an engine output is greater than a predetermined value with an accelerator pedal pressed down by a driver.
Specifically, if the difference in turbine rotational speed increases during upshift where the turbine rotational speed becomes low after shifting, moments of inertial of the turbine and the shift mechanism produce inertia torque which is greatly involved in engagement of the engagement-side friction engaging element. In the technique shown in JP-A 8-145157, considering such inertia torque, the starting supply pressure or initial-stage-engagement pressure is set in accordance with total toque (=turbine torque+inertial torque) acting on the output side of the fluid coupling.
The reference also proposes a technique on upshift control (lift-foot upshift or power-off upshift) carried out when an engine output is smaller than a predetermined value with the accelerator pedal from which the driver removes his/her foot, i.e. when the engine is in the engine-brake state where it is driven by the vehicle or in the coasting state.
Specifically, a basic value of initial-stage-engagement duty ratio is set in accordance with a computed value of turbine torque. During lift-foot upshift, a turbine-torque computed value is equal to roughly zero or a small negative value, so that an initial-stage-engagement duty-ratio basic value is roughly the same regardless of whether the vehicle velocity is high or low. Therefore, an initial-stage-engagement duty ratio is roughly the same regardless of whether the vehicle velocity is high or low.
However, when an initial-stage-engagement duty ratio is roughly the same regardless of whether the vehicle velocity is high or low, a hydraulic actuating member, such as a clutch piston, for engaging the engagement-side friction engaging element such as a wet multiple-disc clutch has different strokes between low and high vehicle velocities. Specifically, at low vehicle velocity, since a difference in rotational speed produced up to synchronization is small in the engagement-side friction engaging element, a time required from supply of the engagement pressure to synchronization is short. On the other hand, at high vehicle velocity, since the rotational-speed difference produced up to synchronization is large, a time required from supply of the engagement pressure to synchronization is long.
When the time required up to synchronization is short, the stroke or movement of the hydraulic actuating member lags behind the synchronizing timing of the engagement-side friction engaging element, such that the engagement-side friction engaging element is engaged after synchronization, i.e. after overshooting synchronization. Such engagement lag leads to an occurrence of drive system shock. On the other hand, when the time required up to synchronization is long, the stroke of the hydraulic actuating member advances with respect to the synchronizing timing of the engagement-side friction engaging element, such that the engagement-side friction engaging element is engaged before synchronization. Such engagement advance also leads to an occurrence of drive system shock, thereby providing a vehicle ejecting feel to the driver.